As is well known, it is much easier and less expensive to construct a large offshore structure on land and tow it to the site for subsequent installation than it is to construct the structure at sea. Because of this, every attempt is made to decrease the amount of offshore work that may be needed in an effort to minimize the cost of the structure. Regardless of these efforts, however, a certain amount of offshore work will still be required in each case.
In the past, when the deck of a large offshore platform was to be installed, it was often found desirable to build the deck as one large component and install it fully assembled by lifting it from the tow barge and placing it upon the substructure. Unfortunately, as the decks became larger and heavier, there were fewer heavy-lift cranes that could handle such a load. Should the deck became too large or too heavy, it was divided into smaller components that were then each individually lifted into place. This prolonged the installation process since multiple lifts were now required and, once installed, the various equipment upon the different components had to be inter-connected and tested, thereby necessitating a large amount of offshore work.
An alternate method to dividing the deck into smaller components, was to build the deck as a complete unit on shore and then skid this oversized deck onto a relatively narrow barge. The barge would then be transported to the installation site where it would be maneuvered between the upright supports of the substructure (thus the need for a narrow barge and for a wide gap between the upright supports of the substructure). Once in place, the barge would be selectively ballasted causing it to float lower in the water thereby enabling the deck to come to rest upon these upright supports of the substructure. Afterwards, the barge would be moved out from under the deck and de-ballasted. Unfortunately, this method necessitates a specially designed substructure with a large open area in its central region near the waterline in order to accept the barge. Normally, such a method is used only for decks which are too heavy to lift in one piece with available heavy-lift cranes. This method also requires a barge that has sufficient beam (width) to provide stability against roll whenever the deck is supported upon the barge. However, to acquire such stability, a wide barge is needed which necessitates an even wider opening in the center of the structure onto which the deck is to be placed which, in turn, results in a longer deck span between the supports of the substructure. Thus, the structural efficiency of both the deck and the substructure is reduced which results in this method only becoming practical for very wide decks and for substructures with reduced deck loads thereon.
Additionally, the manner of ballasting the vessel prior to transferring the deck onto the substructure posed problems. These arose because such ballasting had to occur rather quickly, almost instantaneously, while the deck was properly located and aligned with respect to the substructure. Any sudden wave or wind force could cause such alignment to go astray or the vessel's heave could cause damage to the deck.
It is thus an object of this invention to provide a manner of installing decks upon offshore platforms without requiring the need for heavy-lift cranes or the like. Another object of this invention is to provide an installation method for decks without having to divide the deck into smaller components. Still another object of this invention is to allow selection of a transport vessel of sufficient beam to provide adequate stability against roll. Yet another object of this invention is the ability to install the deck upon a variety of different substructures, there being no need for special configurations thereof. A further object of this invention is to provide a means of rapidly ballasting the vessel during the transfer operation such that the transfer rapidly occurs thereby minimizing both potential mis-alignment and damage to the deck. These and other objects and advantages of this invention will become obvious upon further investigation.